The present invention is related to the use of shape memory materials in medical devices.
A shape memory material can assume an initial high temperature configuration and a deformed low temperature configuration and revert back to the initial high temperature configuration upon the application of heat. The high temperature configuration of the material is set into the material or memorized during an initial shaping step in which the material is maintained at a high temperature in a desired shape for a period of time.
This shape memory exhibited by metals results from the metal undergoing a reversible solid state phase transition. Specifically, the metal transitions from an austenitic state to a martensitic state with a decrease in temperature. The temperature at which the transition begins is typically designated Ms (martensite start temperature) while the temperature at which the transition finishes is designated Mf (martensite finish temperature). As the metal transitions from the austenitic state to the martensitic state it becomes more easily deformed. In the martensitic state, the metal is able to accommodate significant plastic deformation at an almost constant stress level.
Upon heating a metal in the martensitic state, the metal begins to return to an austenitic state. The temperature at which this transition begins to occurs is designated As (Austenitic start temperature). The transition is complete at a temperature designated Af (Austenitic finish temperature). When Af is attained, the metal has fully reverted to its initial, high temperature configuration.
Shape-memory materials have been disclosed in U.S. Pat. No. 3,012,882 to Muldawer et al. and U.S. Pat. No. 3,174,851 to Buehler et al. both of which are incorporated herein by reference in their entirety.
A variety of materials exhibit shape memory properties including binary metals such as Nickel-Titanium alloys including Nitinol. Doped Nickel-Titanium alloys may also exhibit shape-memory properties.
The use of shape-memory metals in medical applications has been disclosed in a number of references including U.S. Pat. No. 5,197,978 to Hess, U.S. Pat. No. 5,540,712 to Kleshinski et al., U.S. Pat. No. 5,597,378 to Jervis, U.S. Pat. No. 5,769,796 to Palermo et al., U.S. Pat. No. 5,846,247 to Unsworth et al. The contents of the above patents are incorporated herein in their entirety by reference.
Shape-memory materials have been used, inter alia, in the production of stents. Nitinol stents which are fully expanded in the austenitic state and compressed or partially expanded in the martensitic state have been disclosed. The specific Nitinol alloy is chosen such that the stent will be in the austenitic state at body temperature. Prior to insertion into the body, the stent is maintained at low temperature corresponding to a temperature within the martensitic range. Upon delivery to a desired bodily location, the stent is warmed to at least the Af temperature and in the process expanded to its desired final diameter.
Shape-memory metals have been disclosed for use in other medical devices as well.
The physical properties, including elasticity and stiffness, of a shape-memory metal may be controlled via a variety of factors including the chemical composition of the alloy and the treatment to which the alloy is subjected. The physical properties of a shape-memory material may also be controlled by heat treating the material. By selective local heat treatment of a shape-memory material, it is possible to destroy the austenitic-martensitic transition and/or change the elasticity and stiffness.
The present invention is directed to medical devices which use shape-memory materials that have been differentially heat treated to exhibit spatially varying properties such as elasticity and stiffness.
The present invention is directed to medical devices having been formed, at least in part, from shape-memory materials which have been locally treated to alter their memory characteristics relative to portions of the device which have not been subjected to the local heat treatment. Specifically, a region of the shape-memory material is treated to decrease the stiffness of the region relative to the remainder of the medical device. Desirably, the local treatment will destroy the superelastic properties of the locally treated region.
The invention is directed in particular to a stent having been formed at least in part of a shape-memory material. The stent comprises a plurality of cells arranged in columns extending from one end of the stent to the other. Adjacent columns of cells are interconnected. The stent includes at least one differently treated column of cells which has been treated differently from the remainder of the stent to have different shape-memory properties from the remainder of the stent.
The invention is also directed to a stent wherein at least one of the columns is characterized by a reduced elasticity relative to the remainder of the columns.
The invention is also directed to a stent wherein at least one of the columns is characterized by a reduced stiffness relative to the remainder of the columns.
The invention is further directed to a stent wherein at a desired temperature range, at least one longitudinal column of cells does not have superelastic properties while the remainder of the longitudinal columns do have superelastic properties.
The invention is also directed to a stent having been formed of a shape memory material and comprising a plurality of spaced segments. Adjacent spaced segments are connected to each other by one or more connector sections. Each segment comprises a series of interconnected cells. Desirably, the cells in plan view are of polygonal configuration. The segments have been subjected to a different heat treatment than the connector sections.
The invention is also directed to a stent having been formed of a shape memory material and comprising a plurality of spaced segments with adjacent spaced segments connected to each other by one or more connector sections. Each segment comprises a series of interconnected cells. Desirably, the cells in plan view are of polygonal configuration. The segments have a reduced stiffness relative to the connector sections. Desirably, the segments are not superelastic.
In another embodiment, the invention is directed to a medical guidewire having been formed at least in part of a shape-memory metal. Different portions of the shape-memory metal have been treated differently to have different shape-memory properties. Desirably, at least one portion of the guidewire is subjected to a local heat treatment resulting in a reduced stiffness compared with the remainder of the guidewire. In this way, a guidewire of desired flexibility can be prepared. Via the localized heat treatment, a guidewire may be prepared in which at least a portion thereof retains its shape memory properties while a portion thereof loses its shape memory properties.
In yet another embodiment, the invention is directed to implantable medical filters such as vena cava filter having been formed of shape-memory materials. At least a portion of the shape-memory material is differently treated to alter the shape-memory properties of the material relative to the untreated shape-memory material. Desirably, the differently treated portion will be heat treated and exhibit a greater tendency toward plastic deformation relative to the remainder of the filter.